Unit A Reproduction

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Earthworms are hermaphrodites. All earthworms produce both eggs and
sperm. They cannot fertilize their own eggs, however. To reproduce sexually,
two earthworms attach, exchange sperm, and then separate (Figure 2). The
sperm cells are stored in a special sac until the eggs are ready. Within a few
days, fertilization takes place in a mucus ring secreted by the clitellum, the
thickened band on an earthworm. The ring becomes a cocoon for the fertilized
eggs and slips off in the soil. The baby earthworms hatch about a week later. GO
Figure 2 Hermaphroditic earthworms
reproduce sexually. Two earthworms
attach and transfer sperm to each
other. Both parent earthworms can
produce offspring.
TRY THIS: Two Sexes in One
Skills Focus: observing, measuring, recording
Materials: living earthworms, magnifier (lens or loupe),
dissecting microscope
Earthworms are hermaphrodites. Each organism has both male
and female reproductive systems. In this activity, you will examine
an earthworm and find its reproductive structures.
1. Examine your earthworm specimen. Identify the anterior
(head) end and the posterior (tail) end. The pointed end is the
anterior end. It has an extra lobe of tissue over the mouth.
2. Earthworm structures are located on specific segments. Count
the segments to the clitellum, the obvious swelling on the
earthworm. Make a drawing of your earthworm, and label
the clitellum.
3. Look for openings on the ventral (underside) side of the
earthworm. You may need to use a magnifier. On segment 14,
there is a pair of pores that lead to the female reproductive
system. These female pores may be difficult to find because
they are in the grooves. The male pores on segment 15 are
easier to find. Locate the male pores. Label the male and
female pores on your drawing.
4. Join another group. Line up your two earthworms, anterior to
posterior, so that the ventral surfaces are touching.
A. Where are the male and female pores when the two clitella
are aligned?
B. What are the advantages of being a hermaphrodite?
Figure 3 Adult sponges are fixed to
one location and contain both male
and female reproductive structures.
Aquatic hermaphrodites, such as sponges (Figure 3) and barnacles, are fixed
to one location for most of their lives. These animals often produce sperm and
eggs at different times. This prevents the sperm from fertilizing the animal’s
own eggs, which would produce identical offspring. In sponges, the sperm are
released into the water and are carried by water movement to other sponges.
Fertilization occurs internally after the sperm enter the sponge’s central cavity
through tiny pores. The baby sponges, called larvae (singular: larva), are
mobile and are released into the water. They swim to other locations, anchor
themselves to the bottom, and grow. Barnacles have a penis, which extends
into a neighbouring barnacle to deposit the sperm. The barnacle larvae are
expelled into the water and are carried by water currents to other locations.
84 Unit A Reproduction
NEL